[generic] Don't set the 'title' if it's not defined in the entry (closes #5061)

Some of them may be an 'url' result, which in general don't have the 'title' field.

youtube_dl/extractor/generic.py

@@ -1208,6 +1208,8 @@ def filter_video(urls):
             return entries[0]
         else:
             for num, e in enumerate(entries, start=1):
+                # 'url' results don't have a title
+                if e.get('title') is not None:
                     e['title'] = '%s (%d)' % (e['title'], num)
             return {
                 '_type': 'playlist',